1. Field of the Invention
The present invention relates to synthetic aperture radars and, more particularly, to motion compensation for synthetic aperture radars.
2. Description of the Prior Art
In the prior art, there has been a continuing effort to develop radar systems which are suitable for high resolution application such as ground-mapping and air reconnaissance. Initially, this finer resolution was achieved by the application of pulse-compression techniques to conventional radar systems which were designed to achieve range resolution by the radiation of a short pulse, and angular, or azimuth, resolution by the radiation of a narrow beam. The pulse-compression techniques provided significant improvement in the range-resolution of the conventional radar systems, but fine angular resolution by the radiation of a narrow beam still required a large diameter antenna which was impractical to transport with any significant degree of mobility. Subsequent to the development of pulse compression techniques, synthetic aperture radar techniques were developed for improving the angular resolution of a radar system to a value significantly finer than that directly achievable with a radiated beamwidth from a conventional antenna of comparable diameter.
In the prior art, an equivalent to a large diameter antenna was established which was comprised of a physically long array of antennas, each having a relatively small diameter. In the case of a long antenna array, a number of radiating elements were positioned at sampling points along a straight line and transmission signals were simultaneously fed to each element of the array. The elements were interconnected such that simultaneously received signals were vectorially added to exploit the interference between the signals received by the various elements to provide an effective radiation pattern which was equivalent to the radiation pattern of a single element multiplied by an array factor. That is, the product of the single element radiation pattern and the array factor resulted in an effective antenna pattern having significantly sharper antenna pattern lobes than the antenna pattern of the single element.
Synthetic aperture radar systems are based upon the synthesis of an effectively long antenna array by signal processing means rather than by the use of a physically long antenna array. With a synthetic aperture radar, it is possible to generate a synthetic antenna many times longer than any physically large antenna that could be conveniently transported so that for an antenna of given physical dimensions, the resultant antenna beamwidth of the synthetic aperture radar is many times narrower than the beamwidth which is attainable with a conventioal radar. Due to their synthesis of an antenna length, or aperture, which is much greater than the actual aperture of the physical antenna, radars using this technique have been characterized as synthetic aperture radars. In the most common synthetic antenna case, a single radiating element is used which is translated to take up sequential sampling positions along a line. At each of these sampling points, a signal is transmitted and the amplitude and the phase of the radar signals received in response to that transmission are stored. After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the signals in storage are somewhat similar to the signals that would have been received by the elements of an actual linear array antenna. More precisely, greater resolution is obtainable for a synthetic aperture radar than for a conventional linear array of equivalent length as a consequence of the non-coherent transmission of the illumination from the sampling points of the synthetic aperture radar. The signals in storage are subjected to a corresponding operation to that used in forming the effective antenna pattern of a physical linear array, that is they are added vectorially, so that the resulting output of the synthetic aperture radar is substantially the same as could be achieved with the use of a physically long, linear antenna array.
In generating the synthetic antenna, the synthetic aperture radar signal processing equipment operates on a basic assumption that, as an equivalent to an actual linear array, the radar platform traverses a straight line at a constant speed. In practice, a vehicle carrying the radar antenna is subject to deviations from such non-accelerated flight and it is therefore necessary to provide compensation for these perturbations in straight-line motion. This motion compensation must be capable of detecting the deviation of the radar platform path from a true linear path. In prior art synthetic aperture radars carried aboard aircraft, the flight path perturbations of the aircraft have been detected by an inertial navigation system from which processing equipment aboard the aircraft has computed the deviations from the assumed linear path. However, the complexity and sensitivity of the inertial navigation system and its associated processing equipment has made such motion compensation equipment expensive to build and difficult to maintain. Moreover, many aircraft are not otherwise equipped with inertial navigation systems and, therefore, the inertial navigation system would have to be specially added if a synthetic aperture radar were to be employed aboard the aircraft. Therefore, there was a need for a synthetic aperture radar motion compensation system which could avoid the use of an inertial navigation system thereby affording a relatively inexpensive motion compensation system and permitting the use of synthetic aperture radar aboard aircraft not otherwise equipped with inertial navigation systems.